Digital oscilloscopes generally use raster scan displays to present the activity of electrical signals to their users. Each raster scan display, such as those seen every day on computer screens, consists of a two dimensional array of pixels, with each pixel location being uniquely defined by a row number and column number. The simplest and lowest cost versions of such displays are "single bit" displays, in that the memory from which they derive the information to be displayed only has one bit of intensity information associated with each pixel. In such a display that single bit of information determines whether the pixel associated with it is either "on" or "off", with "on" dictating that a predetermined amount of intensity is to be used to illuminate the pixel and "off" indicating that the pixel is not to be illuminated at all.
The more complex and expensive alternative to a single bit display is a multi-bit display, which can provide variable intensity (also known as "gray-scale") or color variations as a substitute indicator of brightness. The memory locations associated with each pixel of a variable intensity display contain multiple bits of intensity information, indicating the number of varying intensity levels with which they can be illuminated. Like the pixels of single bit displays, those of multi-bit displays can exhibit an "off" or dark state, but instead of one value of illumination, they have multiple values. Typically, the number of values available is 2.sup.N- 1, where N is the memory depth at each address of the raster memory. Thus, for example, a four bit deep raster scan memory can support fifteen levels of partial through maximum illumination, as well as the dark or "off" state. Pixel intensity can also be translated into differing colors, as well as intensity or "brightness".
With this larger amount of data, multi-bit displays can convey more information about the behavior of electrical signal waveforms under observation, particularly if the signal is not perfectly repetitive and therefore has less activity in some portions than others. U.S. Pat. No. 4,940,931 to Katayama et al. for "Digital Waveform Measuring Apparatus Having A Shading-tone Display", hereby incorporated by reference, describes a system for producing digital variable intensity displays.
Typically, digital oscilloscopes acquire information about the behavior of a circuit node by periodically sampling the voltage present at the node. The oscilloscope probe tip is placed in contact with the node and the probe and front end of the oscilloscope precisely replicate the signal, or some predetermined fraction or multiple of the signal, and present it to an analog-to-digital converter. The output of the analog-to-digital converter is a series of multi-bit digital words that are stored in an acquisition memory. Successively acquired samples are stored at sequentially related addresses in the acquisition memory, and are thereby related to a time scale. Those addresses will eventually be converted back to a time scale, one of which is represented as horizontal distance along the x-axis of the oscilloscope's raster scan display.
In a typical digital oscilloscope, voltage amplitude values derived from the data contents of an acquisition memory location determine the vertical location (row number) of an illuminated pixel, while time values derived from the addresses of the acquisition memory determine the horizontal location (column number). The process of expanding the contents and addresses of an acquisition memory to produce contents for a two dimensional raster memory is known as "rasterization".
The output of a rasterization process is usually combined with some preexisting content of a raster memory, and the resulting composite raster contents may thereafter be regularly subjected to some sort of decay process. For more information about digital persistence and decay, refer to the following U.S. Pat. Nos., hereby incorporated by reference: 5,440,676 to Alappat et al. for "Raster Scan Waveform Display Rasterizer With Pixel Intensity Gradation"; 5,387,896 to Alappat et al. for "Rasterscan Display With Adaptive Decay"; 5,254,983 to Long et al. for "Digitally Synthesized Gray Scale For Raster Scan Oscilloscope Displays".
For any particular combination of settings of the oscilloscope display and acquired waveform data, there will be some function that maps the acquired data points into the time (x-axis) versus voltage (y-axis) display raster. That mapping function will include some ratio between the number of samples to be mapped and the number of pixel columns in the raster display. While that ratio can be 1:1, it will usually be N:1 or 1:N If there are more data points than columns of pixels into which they must be mapped, some form of data compression and/or decimation will be utilized. Decimation means discarding all but every Nth data point, thereby forgoing part of the available information. Compression, on the other hand, means mapping data from multiple time locations in the acquisition memory into one horizontal location, i.e., a single column of pixels, in the raster scan display. If there are fewer data points than columns of pixels, i.e., the 1:N case above, then some sort of interpolation or equivalent time sampling is generally used. In the case of the present inventions, equivalent time sampling, which will be discussed in much greater detail below, is used.
For many years, digital oscilloscopes were limited in the percentage of activity at the probe tip that they effectively processed and displayed for the user. Although less sophisticated users and those who were only familiar with analog oscilloscopes had the impression that they were seeing most or all of the activity at their digital oscilloscope's probe tip, in many circumstances the display was really only showing a small percentage of the actual activity occurring there. This was because these oscilloscopes spent a lot more time processing signals than they did acquiring them. If a signal is perfectly repetitive, this loss of "live time" is not a problem, since one waveform looks just like another. However, when a signal is displaying some sort of intermittent anomalous behavior, a low percentage of live time may prevent such an anomaly from being detected. Therefore, increasing the waveform throughput and the percentage of signal activity at the probe tip that can actually be observed by the user has increasingly been the goal of the more recent digital oscilloscope designs. U.S. Pat. No. 5,412,579 to Meadows, et al. for "Slow Display Method for Digital Oscilloscope With Fast Acquisition System", hereby incorporated by reference, describes an oscilloscope system in which acquisitions are composited into alternating (also known as "ping-ponging") display buffers, so that while the contents of one display buffer is being used as the source of data being displayed, the other one is being used to gather and composite more data. However, the slow display of the design described in this patent only provided a single bit of intensity data per pixel and therefore had no analog-like gray scaling, i.e., variable intensity, capability.
The capability of processing a large number of waveforms is a highly desirable feature in a digital oscilloscope. U.S. Pat. No. 5,530,454 to Etheridge et al. for "Digital Oscilloscope Architecture For Signal Monitoring With Enhanced Duty Cycle", herein incorporated by reference, describes an oscilloscope that is capable of acquiring up to 400,000 waveforms per second when each waveform record is short and the frequency of trigger events is high enough. This oscilloscope somewhat emulates analog oscilloscope performance with a "live time" percentage that varies from about 6% to nearly 99%, depending on the timebase settings, number of channels turned on, and trigger event availability.
The speed of the architecture described in the Etheridge `454 patent was attained by using two raster memories, one in the acquisition system and the other in the display system. The first of these raster memories, referred to as the "raster acquisition memory", was almost constantly supplied with acquired waveforms. These waveforms were rasterized and composited into this raster acquisition memory nearly as quickly as they were acquired by a high speed acquisition rasterizer and similarly rapid image combiner. After many waveforms had been thus composited, the contents of the single-bit per pixel raster acquisition memory was transferred to a multiple-bit-per-pixel display raster memory, where it was combined with previously displayed data. The contents of that display raster memory were subject to a digital persistence control, such as that described above, and the intensity of each pixel was thereby decayed over time.
For many users, especially those having some experience with analog oscilloscopes, variable brightness usefully communicates information about the activity of the signal being observed. Many of these users have had a strong preference for some behaviors that resemble those of analog oscilloscopes. For example, as an analog oscilloscope generates vertical excursions during a horizontal sweep interval to provide a real-time picture of the signal activity at the probe tip, they inherently tend to vary the brightness of the display as an inverse function of the slope of the line they produce. This occurs because the cathode electron gun of the CRT generates a constant supply of electrons that depends on the setting of a "brightness" control, and the length of the trajectory covered in a unit of time is minimally determined by the x-axis distance associated with any particular sweep speed, but is increased by any and all y-axis excursions. And a y-axis excursion can be a large multiple of the corresponding x-axis distance, so the constant available electron beam energy can be reduced by a large factor as it is spread over this much longer distance. Thus, analog oscilloscopes inherently vary the brightness of the line they draw as an inverse function of the slope of that line.
Another even more highly desired feature of an analog oscilloscope or a digital oscilloscope with a high waveform throughput, is the ability to detect an intermittent signal anomaly that occurs in an otherwise repetitive signal. Older digital oscilloscopes, with low "live time" make observing intermittent signal activity improbable, at least in the absence of special trigger modes designed to detect certain classes of intermittent signal activity. Analog oscilloscopes will show a faint trace indicating the presence of this intermittently anomalous signal behavior. Of course, if the signal becomes too intermittent, the trace will be so faint in brightness that it may be missed entirely by the oscilloscope operator.
Sometimes, trigger rates are variable in a way the prevents full utilization of the acquisition and rasterization circuitry. What is desired is to be able to operate this high speed and high volume acquisition and rasterization apparatus in a suitable and efficient manner in the presence of low frequency input trigger rates while maintaining a high probability of capturing the data associated with those slow triggers